Honest raptors refuse to lie about their fitness

The black kite, a species of raptor, prefers plastic to paper when decorating …

Many animals use signals to indicate their general level of fitness, giving potential suitors a hint of how their offspring might fare. For the most part, the effort and energy involved in producing these signals provide an accurate read on fitness. But other species use indirect signals to advertise themselves to mates, which leaves open the possibility of cheating, of advertising a fitness that isn't really there. Researchers have now identified a predatory bird that advertises its fitness using white plastic (no paper, please), and is so scrupulously honest that, when researchers added some additional plastic, most of the birds removed it.

The bird in question is the black kite. The kite chooses its nesting location based on the quality of the nearby food sources, and then defends it, as other kites will challenge a breeding pair for a prime location. The best locations tended to be occupied by fit breeding pairs, to the exclusion of juveniles or older animals.

The two decorative extremes in black kite nests.

F. Sergio

At least in their study area, some of the kites also had a somewhat unusual habit: they filled their nest with pieces of white plastic. The birds were actually pretty picky about this. When offered a variety of colors, they consistently picked white, and vastly preferred to have plastic rather than paper. Whether they've figured out it has greater durability or just like the feel of plastic wasn't obvious. What was obvious was that the plastic didn't do the birds any favors when it comes to basic survival. When the researchers set up fake nests, the ones lined with white plastic suffered a higher degree of predation than the undecorated versions.

There were some hints, however, that the plastic might serve as an indication of fitness. The kites that decorated their nests spent more time eating than their peers, and were better at driving off intruders that might want to steal their nesting site. But they also had to spend less time doing the defending, as highly decorated nests suffered fewer problems with intruding kites.

That last fact suggests that the plastic might act as a signal to other kites, saying to stay away. And it seems that's how the kites view it. The rates of decoration increased with the body condition of the birds doing the decorating, and also correlated with nest site quality. It also had what they describe as a "parabolic" relationship with age. It went up as the kites approached the age of 20, and then dropped afterwards, so that few older birds bothered to decorate.

So, the authors conclude that the plastic is used to advertise the nest owners' fitness, warning other kites that, although the nest is in a prime feeding area, they better be ready for a fight if they want to take it over. Less fit birds avoid this advertising, lest they invite a challenge they're likely to lose.

That last point was driven home by an experiment in which the authors tested the honesty of the birds by adding more plastic. If a mating pair was near the prime age, they typically left the added plastic alone. But 87 percent of the pairs that were younger or older removed it. The birds apparently engage in honest advertising, presumably as a way of avoiding inviting a fight that they can't win.

The technical term for this sort of signaling is "extended phenotype." A phenotype is a visible trait caused by a genetic factor, and the nest decorations "extend" the concept beyond the organism and into its surroundings. Most of the extended phenotypes we're aware of involve some aspect of obtaining a mate, so these new results appear to be the first that show they can apply to resource defense. It would be interesting to find out whether the behavior really is genetic, or whether it's learned socially, a feat birds are obviously capable of. I'm also curious as to whether the same thing applies to the preference for plastic.